The increased number of children who are immunologically altered, either by disease or drug therapy, has caused the accumulation of a large group of children at risk of developing severe "progressive" varicella infection. The objectives of this research proposal are to define factors which determine the outcome of varicella infection. The pathogenesis and course of infection in "normal" and "immunologically altered" children is to be examined, employing viral isolation, measurements of specific humoral response, and an assay of lymphocytoxicity to varicella-infected cells. During the course of varicella, cultures of buffy coat in human fibroblast tissue cultures have revealed an association of viremia with the development of "progressive" varicella. The nature of this viremia is to be evaluated and mechanisms by which viremia is modified is to be investigated.